Systems that use flexible tubes to transfer fluids from one location to another frequently take advantage of the compressibility of the tubes to adjust, restrict or stop fluid flow within them. This is particularly useful in medical applications in which biological or therapeutic fluids are transferred between a source or destination and a patient by means of an electro-mechanical instrument. In hemodialysis, for example, blood is pumped from a patient's body through a dialyzer to remove wastes and excess fluid from the blood. Blood is removed from the patient, pumped through the dialyzer, and then returned back to the patient in a closed-loop system. External blood loss can occur if the intravenous (IV) needle (e.g. a fistula needle) returning the treated blood is removed or dislodged from the patient's body and the blood pump is not stopped in a timely manner. Generally, a dislodgement of an arterial needle or catheter (in which blood is being drawn to the machine from the patient) can be readily detected by the machine's air-in-line detectors. It is much more difficult, however, to detect dislodgement of a venous needle or catheter. This risk is heightened when a patient is asleep or otherwise unaware that tension applied to the intravenous line has resulted in dislodgement of the IV needle. Current dialysis systems cannot readily detect when an indwelling venous IV needle delivering blood from the dialysis machine to the patient has become dislodged from the patient's body. However, such machines can detect an increase in the back-pressure on the pump discharge line, typically from an occlusion or kink in the blood tubing downstream of the pump, which can trigger an alarm and pump shutdown. A device that can compress the flexible tubing transporting fluid to the patient can provide the necessary increase in intraluminal pressure, or possibly even provide complete occlusion, that can prevent excessive fluid loss from a dislodged catheter. The problem is how a threatened intravascular catheter dislodgement can be made to trigger such an occluding device.
It is therefore an object of this invention to provide a device to occlude any flexible tubing carrying a fluid, in which the device can be triggered by the application of a pulling or traction force on a segment of the flexible tubing, the pulling force being sufficient to cause movement of the tubing (if there is slack in the tubing) or elastic stretching of the tubing (if there is no slack in the tubing). In medical applications, such as, for example hemodialysis applications, either the blood tubing connected to the dialyzer or the tubing connected to the intravascular catheter or needle can serve as such a segment of flexible tubing. When a predetermined threshold amount of pulling force occurs along a segment of the tubing between the blood pump and the IV needle, a portion of that segment of tubing will either move or stretch in an amount sufficient to trigger an attached occluding device. The threshold tension required to trigger the device can be set to be less than the force that could cause dislodgement of an indwelling needle or catheter assembly that has been taped to a person's arm or other part of the body. Since dialysis machines are generally programmed to detect an occlusion on the pump discharge tubing, the invention can be easily integrated into an existing dialysis system by locating the occluder on the tubing between the blood pump and the IV needle, thus reducing the risk of accidental exsanguination.